1. Field of the Art
This invention relates to an optical detector for optically detecting the presence of an object.
2. Prior Art
A circuit configuration as shown in FIG. 1 (Prior art) has been generally well known as an apparatus for optically detecting the presence of an object. This circuit is designed so that an infrared-ray emitted when a predetermined voltage is applied through a resistance to an anode of an infrared-ray light emitting diode 1 (hereinafter referred to as "light emitting diode") having a grounded cathode, is received by a phototransistor 2 causing an electric photocurrent to increased in a collector thereof grounded through a resistance R1. This current produces a voltage output across resistance R1 which is detected to determine whether the object is present between the light emitting diode 1 and the phototransistor 2.
Disturbance light of an artificial or natural illumination under which the circuit is used, is, however, detected by the phototransistor 2 to thus increase a collector current, displacing an operative point of the phototransistor 2. This involves disadvantage in that sensitivity for sensing the light from the light emitting diode 1 is lowered.
Due to a noise current derived from the disturbance light, electronic devices such as a computer and the like are likely to malfunction when the output from the circuit is directly connected thereto. For prevention of such malfunction, a complicated analog circuit has been required to eliminate noise the output current from of phototransistor 2.